Compressor,
Turbine, heat exchanger for heating the working fluid termed as heating chamber
and heat exchanger for cooling the working fluid termed as cooling chamber are the
main components of closed cycle gas turbine engine.

Open cycle
gas turbine engine could be modelled as closed cycle gas turbine engine.
Combustion process will be replaced here by constant pressure heat addition
from an external source in heating chamber and discharge process will be
replaced by constant pressure heat rejection in cooling chamber.

Let us see the arrangements of
various components of Brayton cycle or Joule cycle

Air will enter
in to the compressor, where pressure and temperature of air will be increased.
Now air at high pressure and high temperature will enter to the heating chamber
as shown in above figure.

Working
fluid i.e. high pressure and high temperature air will be heated from an
external source in heating chamber. High temperature nuclear rods are used here
for heating the working fluid i.e. air. Hence working fluid i.e. air will have
high pressure and high temperature at the discharge of the heating chamber.

High
pressure and high temperature air will enter in to the turbine, where high
pressure and high temperature air will be expanded through the turbine.
Pressure and temperature of the air, both will be dropped here.

There will
be drop in temperature of air but still temperature of air will be high, while
pressure of air will be reduced up to the pressure at which air will enter in
to the cooling chamber.

Air will
be cooled in to the cooling chamber at constant pressure up to its original
temperature with the help of continuous circulating cold water and hence heat
will be rejected here at constant pressure. Again cold air coming from cooling
chamber will enter to compressor for repeating the cycle.

As we can
observe here that exhaust air is not rejected to atmosphere but also exhaust air
re-circulated to the cooling chamber and therefore this cycle will be termed as
closed cycle gas turbine engine.

Work
energy will be generated from the turbine during the expansion of high pressure
and high temperature air and some part of this generated work will be used to
drive the compressor and hence compressor and turbine are assembled with common
shaft as shown in above figure.

Let us see the processes involved in
Brayton cycle or Joule cycle

Process 1-2: Isentropic compression process, air
entering in to the compressor will be compressed here at high pressure and high
temperature. Pressure will be increased from P1 to P2 and
volume will be decreased here from V1 to V2. Temperature
will be increased from T1 to T2 and entropy will remain
constant as this process will be isentropic process.

Process 2-3: Constant pressure heat addition in
to the heating chamber. Air will be heated from an external source in heating
chamber. Temperature of working fluid i.e. air will be increased here from T2
to T3 and entropy will also increased from S2 to S3.

Process 3-4: Isentropic expansion process, high
pressure and high temperature air will be expanded through the turbine. Pressure
of working fluid i.e. air will be reduced here from P3 to P4
and volume will be increased here from V3 to V4.
Temperature will also be reduced from T3 to T4 and
entropy will remain constant as this process will be isentropic process.

Process 4-1: This process indicates the constant
pressure heat rejection process, where Air will be cooled in to the cooling
chamber at constant pressure up to its original temperature with the help of
continuous circulating cold water. Working fluid i.e. air will be cooled here
from T4 to T1 and entropy will also reduced from S4
to S1.

Let us see here the thermal
efficiency of the Brayton cycle or Joule cycle

As we can see here from PV and TS Diagram, all four processes of Brayton or joule cycle are executed in steady flow devices and therefore
we will analyze these processes as steady flow processes.

We will
see here the various energy calculations for unit mass

Input heat
energy, qin = h3-h2= CP (T3-T2)

Output heat
energy, qout =h4-h1= CP (T4-T1)

Thermal efficiency
of the ideal Brayton cycle will be determined as follow

As we can see that process 1-2 and 3-4 are
isentropic processes and we have also observed here that P2=P3 and P4=P1

Let us substitute above equations into the relationship
of thermal efficiency of ideal Brayton cycle and we will have following
equation.

Where rP
is the pressure ratio and k is the specific heat ratio. We can say from above
equation of thermal efficiency of Braytron cycle that thermal efficiency of Braytron
cycle will be dependent over the pressure ratio and specific heat ratio of the
working fluid.

Do you
have any suggestions? Please write in comment box.

We will
see another topic in our next post in the category of thermal engineering.